1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display apparatus and a driving method therefor.
2. Description of the Related Art
The active matrix type liquid crystal display apparatus controls a transmittance (luminance) of each pixel by a RMS value of a voltage applied thereto. In this liquid crystal display apparatus, as illustrated in FIG. 2, one pixel includes one MOS type transistor. Moreover, the gate is connected to a gate electrode that the pixels in a transverse direction include in common, and the drain is connected to a drain electrode that the pixels in a longitudinal direction include in common. Also, the source is connected to a common electrode that all the pixels include in common and that is positioned on the side opposite to the source with a liquid crystal cell located in between. As illustrated in FIG. 3, the driving method for the display apparatus is as follows: An active state (in FIG. 3, “high”) of a scan line signal, which indicates a scan line to be scanned, is applied to each of the gate electrodes in time-division. In accordance with gray-scale information of display data on the line the scan line signal of which is switched into the active state, a one-level gray-scale voltage is selected out of a plurality of levels, then being applied to the drain electrode. Also, a voltage becoming the reference is applied to the common electrode. This procedure holds, in the respective liquid crystal cells in the line sequence, a gray-scale voltage to be applied at the end of a gate-on state. Namely, it becomes possible to control the applied RMS voltage (luminance) to each pixel in correspondence with display data.
Also, as another driving method, there exists a method disclosed in JP-A-10-54998. In this method, as illustrated in FIG. 4, one pixel includes two MOS transistors. For example, in the first MOS transistor, the gate is connected to the first gate electrode that the pixels in a longitudinal direction include in common, and the drain is connected to a drain electrode that all the pixels include in common, and the source is connected to a drain of the second MOS transistor. Also, in the second MOS transistor, the gate is connected to the second gate electrode that the pixels in a transverse direction include in common, and the source is connected to a common electrode that all the pixels include in common and that is positioned on the side opposite to the source with a liquid crystal cell located in between. As illustrated in FIG. 5, the driving method is as follows: An active state (in FIG. 5, “high”) of a scan line signal, which indicates a scan line to be scanned, is applied to each of the second gate electrodes in time-division. In accordance with gray-scale information of display data on the scan line, a gray-scale voltage control signal with a pulse-width corresponding to the gray-scale information is applied to the first gate electrode. Furthermore, a gray-scale voltage, which is in synchronization with a scanning time-period for one line and has, for example, a ramp waveform, is applied to the drain electrode. Also, a voltage becoming the reference is applied to the common electrode. This procedure holds, in the respective liquid crystal cells in the line sequence, a gray-scale voltage level to be reached at the end of a state where the first and the second gates becomes gate-on simultaneously. Accordingly, as is the case with the former method, it becomes possible to control the applied RMS voltage to each pixel in correspondence with display data.
In the method described earlier, as the number of gray-scales (the number of colors) to be displayed is increased, the number of levels of a gray-scale voltage to be prepared is increased. This condition has resulted in increases in the numbers of gray-scale voltage generating output amplifiers and of gray-scale voltage selecting switches, thereby bringing about a problem of a rising in the cost.
Also, for example, if the above-described method is applied to the liquid crystal display apparatus where peripheral driving circuits and the pixels are formed integrally, it turns out that the above-described output amplifiers and selecting switches are also formed in portions of the peripheral driving circuits. This has resulted in a problem that a variation in the characteristics of these elements gives rise to a deterioration in the picture quality.
Also, in the method described later, the pulse-width of the gray-scale voltage control signal makes it possible to control the transmittance of each liquid crystal cell. This brings about an advantage that, even if the number of gray-scales is increased, there is little increase in the circuit scale. Moreover, since all the peripheral circuits can be configured using digital circuits, there exists an effect of suppressing the above-described variation. In this method, however, two MOS transistors are located within one pixel. This condition causes new problems to occur, such as a decrease in the pixel transmittance and a decrease in the yield.